First published on AO3. Notes taken from there:

"So, I've had this idea for several days now, and couldn't get it out of my head, so here it is.

Also, it's my first fic for the fandom, I hope it's good enough. And I'm veeeeeeery rusty with the whole fanfiction-writing, so bear a little with me.

English's not my first language, and although I'm quite confident with it, I have no beta-reader, so... sorry for any mistakes.

Enjoy?"

It was a stupid thing to do. He knew that from the begining, but it was worth dying young to feel alive for the first time in forever. Even if it was for only a short time. It wasn't so much that he wanted to die, not at all. It all was for the sake of feeling something besides apathy, of saving the boy's life, of doing something better than being hunting for a meaningless, boring job, of being something more.

Then again, that might not matter for long. After that first blow, he was hurting so bad, he might have broken a rib or two. Anyway, he was ready to fall any second, but he wasn't planning on doing it alone. The kaijin raised its claw to deliver another attack and Saitama got ready. As soon as the kaijin stroke, Saitama jumped into the air, tie at the ready to catch the monster's eye, and he was almost there, so close to it he felt like maybe he could win afterall.

Until the other claw closed itself around his middle, the grip strong enough to bring out blood, but not enough to kill. The kaijin looked furious, both its eyes fixing Saitama with a death glare. The rush of excitement that he had first felt died, replaced by a cold fear pooling in his stomach. Not for his life, no. For that glare promised a very painful and very slow death.

Great hero he was. Starting and ending his career in less than an hour. At least he'd endure it all as bravely as he could. The kaijin threw him into the air, and Saitama readied for whatever might happen and the pain it would bring with it.

Except he never landed. A pair of strong, metalic arms held him tightly against a chest of equal features. Saitama opened his eyes, not even aware of when he had closed them, and saw a very young face with dark eyes and golden irises. The kid must had been what?, fifteen? But the look in his eyes reflected a much more mature mindframe, and a confidence a boy his age randomly possesed.

That same boy shifted his grip on the man to secure him in one arm, while he extended his free hand to the kaijin. "Incinerate", the boy breathed in a serious tone. What Saitama witnessed then was the raw power of a blazing attack that ended with the kaijin in a matter of seconds.

Such a destructive power to end an enemy with just one attack, contained in one young body. Saitama's eyes fired up with life, his chest filled with determination and some other emotion he could not place.

"Are you in need of medical assistance?", asked the boy, placing Saitama on his feet now that the inmediate danger was gone.

The adult looked around, the ugly kid must had left long ago during the battle. Wise choice. There was a huge hole burnt where the kaijin used to be and the debris from where he had crashed, a part from that, there was no evidence of what had happened there.

"Sir?"

The boy's voice brought back his attention, and Saitama observed him. He was definitely young, a little smaller than Saitama himself; also a lot less fleshy. His arms were completely mechanical, and as far as what he felt, underneath his clothes there was more metal. But. He was the spitting image of what he himself had wanted to be when he was a kid.

The boy was a hero.

Not a flashy one, that was for sure, but there was an evident air about him that screamed hero. And that was all Saitama needed to come to a resolution.

"What's your name?", Saitama asked, utterly oblivious to the blood running down his sides.

"Genos...", said the boy, an orange glow shining in his eyes. "And you're in need of immediate medical assitance, I can see you have two fractured ribs, a high-rating blood loss, and a possible concussion. Can you state your..."

"Genos! Let me be your disciple!"

Silence fell upon them. A concerned look cross Genos's black-golden eyes; not that it surprised Saitama. He knew how crazy he sounded. But, he wasn't ready to give up his newfound passion, this thing that made him feel alive.

"Please, take care of me, Sensei!"

Extra:

"You seriously need to go to the hospital. You're raving."

"Oh, that reminds me, Sensei. I don't have medical insurance"

"What!?"